Merocyanine dye sensitized photoconductive composition

ABSTRACT

NOVEL NEROCYANINE DYES USEFUL AS PHOTOCONDUCTIVE MATERIAL SENSITIZERS AND SUCH PHOTOCONDUCTIVE MATERIALS SENSITIZED THEREWITH SUCH MEROCYANINE DYES COMPRISE TRINUCLEAR DYES OF NEUTRAL CHARGE HAVING A TERMINAL NUCLEUS COMPRISING THIOBARBITURIC ACID OR A DERIVATIVE THEREOF. A SECOND HETEROCYCLIC NUCLEUS OF THE TRINUCLEAR DYES COMPRISES 4-OXOTHIAZOLE.

United States Patent 3,563,735 MEROCYANINE DYE SENSITIZED PHOTO- CONDU CTIVE COMPOSITIION Salvatore Emmi, Bingharnton, N.Y., assignor to GAF Corporation, New York, N.Y., a corporation of Delaware N0 Drawing. Filed July 15, 1968, Ser. No. 744,658 Int. Cl. G03g 5/08 US. Cl. 96-1.7 12 Claims ABSTRACT OF THE DISCLOSURE Novel merocyanine dyes useful as photoconductive material sensitizers and such photoconductive materials sensitized therewith wherein such merocyanine dyes comprise trinuclear dyes of neutral charge having a terminal nucleus comprising thiobarbituric acid or a derivative thereof. A second heterocyclic nucleus of the trinuclear dyes comprises 4-oxothiazole.

The present invention is directed to novel dyestuffs, photoconductive compositions containing the same for sensitizing purposes, and an improved process of electrostatic printing Wherein a photoconductive composition containing the dyestuff sensitizers of the present invention are employed; more particularly, the present invention relates to dyestuff sensitizers, photoconductive compositions stabilized therewith, and electrostatic printing processes employing the same wherein such dyestuff sensitizers comprise trinuclear dyes of neutral charge having a terminal nucleus comprising thiobarbituric acid or a derivative thereof and a further heterocyclic nucleus comprising 4- oxothiazole.

Electrostatic printing processes for the production of visible records or reproduction are well known in the art being extensively described in the literature both patent otherwise. In general, such processes encompass as the salient features of operation the conversion ofa lightimage or electrical signal into an electrostatic charge pattern on an electrically insulating layer. Image-forming development can thereafter be effected according to any one of several procedures whereby to render the latent charge pattern visible. Electrophotographic processes based upon the utilization of photoconductive layers include of course the Xerographic methods in which an electrically conductive support is first subjected to a uniform electrostatic charge in the dark, this being accomplished for example by means of a high voltage corona discharge whereby an electrostatic charge is created on the element surface. Latent image formation can thereafter be effected by focussing a light-image on the charged surface, the light energy serving to selectively dissipate the electric charge in proportion to the intensity of the incident light radiation, i.e., an imagewise dissipation of the electric charge in accordance with the impressed lightimage. The residual charged areas of the photoconductive layers, i.e., those protected by the image areas of the original and thus unaffected by the exposure radiation provide what is tantamount to a latent electrostatic image pattern which can be readily rendered visible by application thereto of a suitable colorant, e.g., toner powder, having optical density suflicient to permit visible discernment of the image areas and which readily adheres to the residual charged areas. By the foregoing electrostatic development operation there is obtained a permanent visible image which provides an exact replica of the original. It is appreciated of course that a number of ramifications to the aforedescribed basic process by way of improvement have been promulgated in the art; invariably, however, such methods depend for feasible practice upon the principal of light-induced charge-decay whereby to provide the surface of the image-recording member with a residual charge pattern capable of conversion to a readily visually comprehensible image. In general, it is found to be more effective practice to transfer the developed image which for example, may be defined by a pigmented resinous composition constituting a toner powder to a receiving sheet in contradistinction to methods wherein the photoconductive plate itself provides direct means for producing the desired photographic copy absent any transfer operation involving a receiving or master sheet. Photoconductive layers for use in electrophotographic reproduction processes of the foregoing type are conventionally prepared with such photoconductor materials as selenium, cadmium sulfide, zinc oxide, etc. For a plurality of reasons, zinc oxide has proved to be particularly beneficial for the vast majority of operations associated with electrography. The zinc oxide photoconductor material is conventionally employed in photoconductive layers in the manner described, i.e., a grounded support usually paper, is initially rendered sensitive to light by subjecting same to a blanket negative electrostatic charge on the zinc oxide layer in the substantial absence of any ultraviolet or visible radiation. As previously mentioned this step can readily be effected by means of ion transfer from a corona discharge. Following exposure, the resulting latent image areas, i.e., nonlight-struck portions of the photoconductive layer are developed, for example, with a pigmented resin powder having a charge opposite to the negative charge of the unexposed areas of the photocondutive layer. In this fashion, the pigmented powder firmly attaches itself via electrical attraction to s ch negatively charged areas. The strength of adhesion of the resin powder to the image bearing layer can be enhanced by a suitable fixing operation as for eXample by simply heating the resinous material to temperatures sufiicient to fuse or melt same whereby such resin becomes permanently affixed to the surface of the image layer. It is to be understood of course that the temperatures employed in this operation should be selected so as to avoid ny possibility of charring the paper support. In any event, suitable methods for effecting the development of a latent electrostatic image pattern are described in the prior art with recourse to a particular one depending primarily upon the requirements of the processor.

As previously mentioned, particularly beneficial results are obtained with the use of photoconductive layers containing as the photoconductor substance, zinc oxide. Despite the advantageous features inherent in the use of this material, certain problems are nevertheless encountered as regards attempts to impart optimum spectral response thereto. Since the effective photographic speed of the reproduction process vitally depends upon the actinic response of the photoconductor material, the overriding importance of this factor is readily evident. Thus, practically without exception, the commercial grade zinc oxide photoconductor materials provided specifically for use in connection :with the formulation of photoconductive layers exhibit maximum or peak spectral response to but a rather limited region of the spectrum, primarily, the far blue and the ultraviolet. In contradistinction, the vast majority of the light sources customarily employed for electrophotographic exposures display maximum output in the visible spectral region, e.g., an ordinary tungsten light. Such restricted spectral response quite obviously imposes stringent and burdensome limitations upon the process in those instances wherein zinc oxide is employed as the photoconductor material since the corresponding requirement is presented that a light source having the proper radiant emission be employed.

In view of the premier commercial importance of zinc oxide in the electrophotographic industry, considerable industrial activity has centered around the research and development of various means by which to extend the spectral response of zinc oxide photoconductors whereby to impart thereto peak sensitivity in those spectral regions forming the locus of the emissions characterizing those light sources which would ordinarily be employed. At this point it should be mentioned that one suggested remedy to the foregoing problem involves the use of photoconductive materials having a spectral response in the visible spectrum. Photoconductor substances which have heretofore been suggested for such purposes include, for example, the colored oxides, sulfides, selenides, tellurides, and iodides of such materials as cadmium, mercury antimony, bismuth, thallium, molybdenum, aluminum, lead, zinc, etc. Although providing some measure of improvement, procedures based upon the use of the latter photoconductor materials have nevertheless proved somewhat unsuitable for certain applications. Thus, for the most part, the industrial effort thus far exp-ended has been concerned with the development of materials capable of absorbing radiant energy and of transferring the energy so absorbed to the photoconductor. Thus, it has been suggested to incorporate sensitizing dyes with the zinc oxide photoconductor for the purposes of imparting the requisite spectral sensitivity to the reproduction system. Representative dyestuff materials heretofore promulgated in this regard include, for example, rose bengal, eosin, malachite green, crystal violet, methylene blue, methylene grey, fluorescein and the like. Although the use of such dyestuffs has contributed greatly to resolving the problems associated with zinc oxide spectral sensitivity, other problems of a rather significant nature have nevertheless arisen as an incident thereto. Perhaps the primary objection to the sensitizing dyestuffs thus far suggested relates to their pronounced tendency to impart to the sensitizing formulation a spurious off-white tint or coloration thus vitiating to a significant extent attempts to achieve satisfactory contrast, gamma and the like. More specifically, such dyestuffs lead to the formation of tints which may be blue, green, yellow, orange or red as well as various shades and hues thereof. Moreover, as will be appreciated, the undesired coloration of the zinc oxide layer is objectionable from an aesthetic standpoint the latter being a relatively important consideration bearing directly upon the possibilities of commercial acceptance. In some instances, the recording clement itself may be contemplated for further exposure sequence, e.g., the production of either black and white or color prints therefrom. The deleterious effects directly attributable to any spurious tint or off-white shade present in the image bearing layer will, practically without exception, be manifested in the form of inferior photographic quality in the resulting print. Such adverse effects are particularly evident with regard to color reproduction since the presence of spurious coloration in the recording element gives rise to faulty absorption densities, i.e., any fugitive color density will efiectively modulate the exposure radiation and thus to this extent, effect undesired shifts in the color composition, color balance, etc., of the color print.

A further serious objection to a considerable number of the sensitizing dyestuffs thus far proposed concerns their instability under varying conditions of pH. This imposes rather severe limitations on the processors latitude of operations tending to circumscribe severely the range of selection of many of the remaining ingredients to be employed in the coating formulation. For example, the pH hypersensitivity of many of the known sensitizing dyestuffs restricts the processor, for example, in regard to the nature of the resin materials which may be efficaciously employed. For example, if the resin material selected is not correlated with the pH sensitivity characteristics of the sensitizing dye, undesired colorations develop in the coated layer and invariably become exceedingly more pronounced on standing and thus are readily visually perceptible. Apparently, discoloration of the coated element resalts from the interlayer diffusion of acidic or alkaline materials, as the case may be, such conditions being conducive to the creation of spurious, off-white tints. As explained hereinbefore, fugitive coloration of the electrophotographic element is not only aesthetically displeasing but, and perhaps more importantly, renders such element substantially unsuitable for further photocopying operations. Thus, as a concomitant to the use of sensitizing dyestuffs possessed of the pH sensitivity property, it becomes incumbent upon the formulator to adjust or otherwise modify the coating composition by way of compensating for undesired shifts in coloration which would otherwise occur. In many instances, resort to the use of masking dyestuffs, i.e., dyestuffs having spectral absorption subr stantially complementary to that of the fugitive color tint,

is made mandatory. As will be appreciated, remedial techniques of this nature can prove burdensome to the formulator, requiring relatively precise and predetermined adjustments in coating formulae. The costs involved in implementing such techniques may well be prohibitive.

Other problems of equal significance which have been noted to attend the use of a many of the sensitizing dyestuffs thus far provided relate to their suboptimum com patibility with one or more of the ingredients comprising the electrostatic layer coating composition. In this regard, it is imperative to quality reproduction that .the photoconductive coating composition containing the zinc oxide photoconductor be provided, prior to actual coating, in the form of a homogeneous and uniform dispersion of the involved ingredients. Any departures in this connection from optimum uniformity of dispersion render the final coating substantially incapable of uniform spectral response, i.e., the density equivalent of a given exposure product will in all likelihood vary throughout the coated layer. Thus, the possibility that the recording system will necessarily reflect the point-to-point density variations in the original to be reproduced is substantially emasculated.

These inherent deficiencies and disadvantages of prior art photoconductive compositions and sensitizing dyestuffs employed therein have been overcome in accordance with the present invention by the development of a stable, photosensitive dyestuffs which enables the attainment of a high quality reproduction absent the undesirable features characterizing sensitizing dyestuffs previously employed in the prior art.

Accordingly, it is a principal object of the present invention to provide dyestuff sensitizers, photoconductive compositions containing the same, and an improved electrostatic printing process utilizing the same which dyestuff composition and process are free from the inherent deficiencies and disadvantages of the prior art.

A further object of the present invention is to provide an optically sensitized electrophotographic composition or layer having excellent sensitometric properties, e.g., speed, contrast, gamma, etc., such compositions being capable of yielding high quality reproduction in electrostatic printing processes.

A still further object of the present invention relates to optically sensitized electrophotographic compositions or layers having excellent actinic response and stability characteristics, such layers being substantially devoid of any tendency to develop spurious coloration.

A still further object of the present invention is to provide such optically sensitized electrophotographic compositions or layers wherein such compositions contain an optically sensitizing dyestuff comprising a trinuclear heterocyclic dye containing a terminal thiobarbituric acid group and an additional heterocyclic nucleus comprising 4- oxathiazole and derivatives thereof.

Still further objects and advantages of the novel dyestuffs, electrophotographic compositions, and electrostatic printing process of the present invention will become more apparent from the following more detailed description thereof.

The objects and advantages of the present invention as set forth above are obtained in accordance with the present invention by providing trinuclear sensitizing dyestuffs of the general formula:

wherein R and R represent groups selected from alkyl, hydroxy alkyl, alkoxy alkyl, carboxy alkyl, acyloxyalkyl, etc., R and R are selected from the group consisting of hydrogen, alkyl, aryl, etc., and Z represents the nonmetallic atoms necessary to complete a heterocyclic nucleus, 12 being a positive interger of one or two.

Thus, suitable values for R, R R and R include, for example:

alkyl (lower alkyl) methyl ethyl n-propyl n-butyl isobutyl n-amyl isoamyl, etc. hydroxyalkyl ,B-hydroxyethyl 'y-hydroxypropyl fl-hydroxypropyl hydroxyethyl 'y-hydroxypropyl fl-hydroxybutyl, etc. alkoxyalkyl ,B-methoxy methyl ,B-ethoxy ethyl ,B-methoxy propyl -methoxy propyl ,B-ethoxy butyl, etc. carboxyalkyl carboxymethyl -carboxyethyl tt-carboxyethyl -carboxypropyl y-carboxypropyl, etc. acyloxyalkyl [i-acetoxyethyl -acetoxypropyl, etc.

Typical heterocyclic nuclei as defined by Z above include those of the thiazole series (e.g., thiazole, 4-methylthiazole, S-methylthiazole, 4-phenylthiazo1e, S-phenylthiazole, 4,5dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyDthiazole, etc.), those of the benzothiazole series (e.g., benzothiazole 4-chlorobenzothiazole, 5-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4- methylbenzothiazole, S-methylbenzothiazole, 6-methylbenzothiazole, S-bromobenzothiazole, 6-bromobenzothiazole, 4-phenylbenzothiazole, 5-phenylbenzothiazole. 4-methoxybenzothiazole, 5-rnethoxybenzothiazole, 6-methoxybenzothiazole, S-iodobenzothiazole, 6-iodobenzothiazole, 4- ethoxybenzothiazole, S-ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6 dimethoxybenzothiazole, 5,6 dioxymethylenebenzothiazole, S-hydroxybenzothiazole, 6-hydroxybenzothiazole, S-carboxybenzothiazole, etc.), those of the naphthothiazole series (e.g. a-naphthothiazole, finaphthothiazole, S-methoxy-[i-naphthothiazole, S-ethoxy- [3naphthothiazole, 7-methoxy-a-naphthothiazole, S-methoxy-a-naphthothiazole, etc.) those of the thianaphtheno- '7,6',4,5-thia20le series (e.g., 4'-methoxythianaphtheno- 7,6-4,5-thiazole, etc.) those of the oxazole series (e.g., 4-methyloxazole, S-methyloxazole, 4-phenyloxazole, 4,5- diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, 5-

phenyloxazole, etc.) those of the benzoxazole series (e.g., benzoxazole, 5 chlorobenzaoxazole, 5 phenylbenzoxaole, 5 methylbenzoxazole, 6 methylbenzoxazole, 5,6- dimethylbenzoxazole, 4,6 dimethylbenzoxazole, 5 methoxybenzoxazole, 6 methoxybenzoxazole, 5 ethoxybenzoxazole, 6 chlorobenzoxazole, 5 hydroxybenzoxazole, 6 hydroxybenzoxazole, 5 carboxybenzoxazole, etc.) those of the naphthoxazole series (e.g., a-naphthoxazole, fl-naphthoxazole, etc.) those of the selenazole series (e.g., 4-methylselenazole, '4-phenylselenazole, etc.), those of the benzoselenazole series (e.g., benzoselenazole, S-chlorobenzoselenazole, S-methoxybenzoselenazole, S-hydroxybenzoselenazole, tetrahydrobenzoselenaole, etc.), those of the naphthoselenazole series (e.g., ot-naphthoselenazole, B-naphthoselenazole, etc.), those of the thiazoline series (e.g., thiazoline, 4-methylthiazoline, etc.), those of the 2-quinoline series (e.g., quinoline 3- rnethylquinoline, S-methylquinoline, 7-methylquin0line, 8- methylquinoline, 6-chloroquinoline, 8-chloroquinoline, 6- methoxyquinoline, 6-ethoxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline, etc.), those of the 4-quinoline series (e.g., quinoline, 6-Inethoxyquinoline, 7-methylquinoline, S-methylquinoline, etc.) those of the l-isoquinoline series (e.g., isoquinoline, 3,4-dihydroixoquinoline, etc.), those of the 3,3-dialkylindolenine series (e.g., 3,3-dimethylindolenine, 3,3,S-trimethylindolenine, 3,3,7-trimethylindolenine, etc.), those of the Z-pyridine series (e.g., pyridine, 3-methy1pyridine, 4-methylpyridine, S-methylpyridine, 6-methylpyridine, 3,4-dimethylpyridine, 3,5-dimethylpyridine, 3,6-dimethylpyridine, 4,5-dimethylpyridine, 4,6-dimethylpyridine, 4-chloropyridine, 5-chloropyridine, 6-chloropyridine, 3-hydroxypyridine, 4-hydroxypyridine,5- hydroxypyridine, 6-hydroxypyridine, B-phenylpyridine, 4- phenylpyridine, 6-phenylpyridine, etc.), those of the 4- pyridine series (e.g., Z-methylpyridine, B-methylpyridine, 2-chloropyridine, 3-chloropyridine, 2,3-dimethylpyridine, 2,5-dimethylpyridine 2,6-dimethylpyridine, Z-hydroxypyridine, 3-hydroxypyridine, etc.) those of the imidazole and benzimidazole series (e.g., Z-imidazole, 4,5-dimethylimidazole, 4-chloroimidazole, benzimidazole etc.), pyrazole, etc.

Accordingly, it can be seen that the heterocyclic nucleus formed from the radical Z can comprise any and all 5- and 6-membered heterocyclic rings containing a heterocyclic nitrogen atom as the sole hetero element, e.g., those nuclei of the pyridine and quinoline series, etc., or can contain a hetero nitrogen atom with one or more hetero atoms selected from nitrogen, sulfur, oxygen and selenium. As evident from the above, the heterocyclic nucleus may also comprise a fused aromatic ring heterocyclic nucleus wherein such fused aromatic ring is selected from monocyclic and bicyclic aromatic rings.

Also, as noted from the representative heterocyclic nuclei listed above, such nuclei can contain various substituents, e.g., alkyl, aryl, benzyl, halo, alkoxy, hydroxy, etc., which substituents are well-known in the art as is their relationship to dyestuff molecules of the type set forth above. The only requirement of such substituents is that they be substantially innocuous or nonreactive and exhibit no tendency to delteteriously effect the sensitizing properties of the dyestutf molecules.

The following illustrate representative compounds falling within the above-identified generic formula:

(1 2- (4,6-dione-2-thione-5- 1H,3H] -Pyrimidinylidene- 5- [2-(3-ethyl-Z-benzothiazolylidene) ethylidene1-3- ethyl-4-oxothiazoline (2) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene -5- 2- 3-ethyl-2-b enzothiazolylidene ethylidene] -3 -fl-carboxyethyl-4-oxathiazoline (3) 2- 4,6-clione-2-thione-5-[ 1H,3H] pyrimidinylidene 5- 2- 3-B-varboxyethyl-2-benzothiazolylidene) ethylidene] -3-ethyl-4-0xothiazoline (4) 2- 4,6-dionel -phenyl-2-thione-5- 3H] -pyrimidinylidene -5- [2- 3-ethyl-Z-benzothiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline (5) 2-( l,3-diethyl-4,6-dione-2-thione-5-pyrimidinylidene -5- 2-( 3-ethyl-2-benzothiazolylidene)ethylidene] -3-ethyl-4-oxathiazoline (6) 2-(4,6-dione-2-tl1ione-5-[1H,3H]-pyrimidinylidene-S- [2-(3-ethyl-2-benzoxazolyidene)ethylidene1- 3-fi-carboxyethyl-4-oxathiazoline (7) 2-(4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene)-5-[2-(3-ethyl-5,6-dimethyl-2-benzoxazolyidene)-ethylidene] -3-ethyl-4-0xathiazoline (8) 2-(4,6-dione-2-thione-5-[ lH,3H]-pyrimidinylidene -5- [2- 3-5-carboxyethyl-6-methoxy-2-benzothiazolylidene)ethylidene-3-ethyl-4-oxathiazoline (9) 2-(4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene -5- 2-( 3-fi-carboxyethyl-6-methoxy-Z-benzothiazolylidene ethylidene-6-ethyl-4-oxathiazoline (l) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene --[2- (3-B-carboxyethyl-6-methoxy-2-benzothiazolylidene)ethylidene-6-isopropoxy (ll) 2-(4,6-dione-2-thione-5-[ lH,3H]-pyrimidinylidene)-5- [2-(3-fi-carboxyethyl-6-methoxy-2-benzothiazolyidene)ethylidene-5-butyl-4-oxathiazoline (12) 2-( l,3-dimethyl-4,6-dione-2-thione-5-pyrimidinylidene -5- 2- 3-ethyl-2-benzoxazolylidene ethylidene]-3-ethyl-4-oxathiazoline (13) 2-( l,3-diphenyl-4,6-dione-2-thione-5-pyrimidinylidene)-5-[2-(3-ethyl-2-benzoxazolylidene)ethylidene] -3-ethyl-4-oxathiazoline (l4) 2- (4,6-dione-2-thione-5- 1H,3H] -pyrimidinylidene -5 2- 3-ethyl-2-thiazolylidene ethylidene] 3-ethyl-4-oxathiazoline (15) 2-(4,6-dione-2-thione-5- 1H,3H] pyrimidinylidene -5-[2-(3 -ethyl-2-thiazolylidene)ethylidene)- 3-fl-carboxyethyl-4-oxathiazoline (16) 2-(4,6-dione-2-thione-5-[1H,3H] )-pyrimidinylidene-5- 2- 3-fl-carboxyethyl-2-thiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline (17) 2-(4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene)-5-[2-( 3-ethyl-2-oxazolylidene) ethylidene]- 3-fi-carboxyethyl4-oxathiazoline (l8) 2-(4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene -5-[2- (3-ethyl-5,6-dimethyl-Z-oxazolylidene) ethylidene]-3-ethyl-4-oxathiazoline (19) 2- (4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene)-5- [2-(3-ethyl-4-pyridylidene)ethylidene] 3-ethyl-4-oxathiazoline (20) 2-(4,6-dione-2-thione-5-[ 1H,.3H] -pyrimidinylidene -5-[2- 3-ethyl-2-benzimidizolylidene ethylidene]-3-B-carboxyethyl-4-oxathiazoline (21) 2- (4,6-dione-l-phenyl-Z-thione-S-[BH]-pyrimidinylidene 5- 2-( 3-ethyl-2-benzimidizolylidene)ethylidene]-3-ethyl-4-oxathiazoline (22) 2-(4,6-dione-2thione-5-[1H,3H]-pyrimidinylidene) -5-[2- (3 -,8-carb oxyethyl-2-naphthothiazolylidene ethylidene] -3-ethyl4-oxothiazoline (23) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidenc)-5- [2, (3-ethyl-5,6-dimethyl-Z-naphthoxazolylidene)-ethylidene]-3-ethyl-4-oxathiazoline (24) 2-(1,3diphenyl-4,6-dione-2-thione-5-pyrimidinylidene)-5-[2-(3-ethyl-2-quinolidenc) ethylidene] -3- ethyl-4-oxathiazoline (25) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene -5- 2- 3-[3-carboxyethyl-6-;8-methoxyethyl-2- benzothiazolylidene)ethylidene-6-ethyl-4-oxathiazoline (26) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene -5- 2- 3-B-carboxyethyl-6-fi-methoxyethyl- 2-benzoselenazolylidene)ethylidene-6-ethyl-4- oxathiazoline (27) 2-(4,6-dione-2-thione-5-[lH,3H]-pyrimidinylidene -5-[2-( 3-hydroxyethyl-2-benzothiazolylidene) ethylidene]-3-ethyl-4-oxathiazoline (28) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene)-5-[2-(3-[5-ethoxy ethyl-2-benzothiazolylidene) ethylidencl -3-6-carboxyethyl-4-oxathiazoline (29) 2-( l,3-diethyl-4,6-di0ne-2-thione-S-pyrimidinylidene)-5-[2-(3-fi-acetoxyethyl-2-benzothiazolylidenc) ethylidene]-3-ethyl-4-oxathiazoline (30) 2-(4,6-dione-2-thione-5-[1H,3H1-pyrimidinylidene)-5-[2-(3-ethyl-2-benzothiazolylidene)ethylidene]-3-B-ethoxyethyl-4-oxathiazoline (31 2-(1,3-diethyl-4,6dione-2-thione-S-pyrimidinylidene) -5- (2-( 3-ethyl-2-benzothiazolylidene) ethylidene]-3-fi-methoxyethyl-4-oxathiazoline (32) 2-(4,6-dione-2-thione-5-[lH,3H]-pyrimidinylidene -5-[2- 3-ethyl-2-thiazolidinylidene)ethylidene]-3-ethyl-4-oxathiazoline (33) 2-(4,6-dione-2-thione-5-[lH,3H]-pyrimidinylidene -5- [2- 3 -ethyl-2-thiazolidinylidene ethylid ene] -3-ca rb oxymethyl-4-oxathiaz0line (34) 2-(4,6-dione-2-thione-5-[1H,3H]pyrimidinylidene-S- [4-( 1-phenyl-3 -methy1-5-pyrozolan-4- ylididene) methyl] -3-ethyl-4-hydroxy-2- thiazolylidene The above trinuclear dyes useful as dyestufi sensitizers in photoconductive systems and photoconductive reproduction processes are prepared by first alkalating a dinulcear dye, e.g., merocyanine, oxonol, or styryl dye, containing a rhodanine nucleus and subsequently reacting such dinuclear dye with thiobarbituric acid. In this respect, the process of preparing the trinuclear dyes of the present invention is generally that set forth in preparing trinuclear dyes in general, such methods of preparation being described in the prior art. Thus, for example, among the various patents describing the method of preparation of trinuclear dyes are US, Pat. 2,442,710 to Riester, US. Pat. 2,388,963 to Fry et al., and US. Patent 2,454,629 to Brookcr.

Thus, for example, in the preparation of the dye 2 (4,6 dione 2 thione 5 [1H,3H] pyrimidinylidene) 5 [2 (3 carboxymethyl 2 benzothiazolylidene) ethylidene] 3 ethyl 4 oxathiazoline, the dinuclear merocyanine dye useful as a starting material in the production of the trinuclear dye is prepared by reacting 2 ethyl 4 anilinovinylrhodanine with a substantially equivalent amount of 2 methyl 3 carboxyethylbenzothiazolium iodide. After refluxing for a period of 15 minutes to 30 minutes in the presence of a methanol solvent, acetic anhydride and triethyl amine, the reaction mixture is cooled and filtered to yield a crystalline precipitate of the desired merocyanine dye in yields ranging from 60 to The trinuclear dye of the present invention is prepared by reacting the merocyanine dye produced as described above with methyl para-toluene sulfonate under reflux conditions and, after allowing the alkylated merocyanine dye to cool, the alkylated dye is reacted with thiobarbituric acid in 21 methanol solvent in the presence of triethylamine to prepare the desired trinuclear dye. Thus, in accordance with the production of the exemplary material as noted above, such a process yields a pure dye which exhibits an absorption maximum in methanol at 585 m.

By varying the starting material in accordance with the desired trinuclear dye, all of the trinuclear dyes falling within the generic formula of the present invention can be prepared by the general process outlined above. Again, the process for producing the trinuclear dyes of the present invention is essentially that as set forth in the prior art for preparing trinuclear dyes in general.

One of the particularly surprising discoveries of the present invention concerns the fact that dyestuffs of the type described above not only exhibit an exceptionally high order of sensitizing efficiency, i.e., impart a high order of spectral response to zinc oxide photoconductorcontaining photoconductive layers, but in addition, are highly stable over varying conditions of pH thus removing any limitation as regards the nature of the resin binder material employed. In addition, their compatibility with the various other ingredients conventionally employed in photoconductive coating compositions presents significant advantage. Such dyestuffs are further atypical in that photoconductive elements containing same may be stored for extended periods of time either before or after electrophotographic processing in the virtual absence of discoloration whether due to pH or other conditions.

The dyestulf product is preferably employed in relatively small amounts, i.e., proportions ranging from about to about 75 milligrams per pound of zinc oxide with a range of from about 18 to about 24 milligrams being particularly preferred. Such proportions are not critical per se merely encompassing those values found to assure the obtention of optimum results. The requirements of a particular process may well dictate the propriety of de partures therefrom. It will be understood that such dyestuffs may be employed singly or in admixture comprising 2 or more depending primarily upon the requirements of the processor, e.g., the peak sensitivity values desired in the photoconductive layer.

The photoconductive layer compositions of the present invention may be prepared according to conventional procedures described in the prior art, i.e., utilizing conventional solvents, coating aids, driers, etc., such ingredients being of an optional nature. In any event, the essential components of the photoconductive composition comprise the sensitizing dye, the photoconductor material, i.e., zinc oxide, the latter being dispersed in an insulating binder material having relatively high dielectric strength and good electrical insulating properties. As particular examples of film-forming insulating binders found to be suitable for use herein there may be mentioned the following: styrenebutadiene copolymers; silicone resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride; acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate; vinyl chloride copolymers; poly(vinyl acetals); such as poly (vinyl butyral); polyacrylic and methacrylic esters, such as poly(metfihyl methacrylate), poly (n-butylmethacrylate), poly (isobutyl methacrylate etc.; polystyrene, nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly(ethylenealkaryloxyalkylene terephthalate); phenol-formaldehyde resins; ketone resins; polyamide; polycarbonates; etc. Methods of making resins of this type have been described in the prior art, for example, styrene alkyd resins can be prepared according to the method described in US. Pats. 2,361,019 and 2,258,423. Suitable resins of the type contemplated for use in the photoconductive layers of this invention are sold under such trade names as Vitel PE- 101-X, Cymac, Piccopalo 100, and Saran F220. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paratfin, mineral waves, etc. Other polymeric materials found to be especially advantageous include, for example, a polyester material available commercially from the Celanese Corporation of America under the commercial trade name designation Epitex 1311, which is prepared by reacting epichlorohydrin with bisphenol A employing the former in slight molar excess and thereafter reacting the polyether obtained with a mixture of a dimerized fatty acid and soya fatty acid. The resultant product comprises a linear, acetone-soluble, nonheat curable polymer material containing epoxy groups. Methods for the preparation of such polymers are described, for example, in U.S.P. 2,970,983. A further resin material found to be admirably suited for use herein comprises a product available commercially from the Pennsylvania Industrial Chemical Company under the trade name designation Piccolastic which is identified as being a low molecular weight (on the order of approximately 400), low melting point (approximately 75 C.) polystyrene resin. In accordance with the present invention, it has been ascertained to be especially advantageous to employ the resinous polymeric materials in admixtures comprising 2 or more and thus to capitalize on the superior properties which may typify 10 specific ones. The term resinous binder as used herein is thus to be accorded a significance consonant therewith, i.e., extending to either the singular or conjunctive use of such resin materials.

Suitable solvents for effecting homogeneous dispersion of the ingredients comprising the layer composition include, for example, toluene, xylene, benzene, acetone, 2- butanone, chlorinated hydrocarbons, e.g., methylene chloride, ethylene chloride, etc., ethers, e.g., tetrahydrofuran or mixtures of such solvent materials.

Alternatively, the ingredients comprising the photoconductive coating composition may be provided in the form of an aqueous system in contradistinction to an Organic solvent system. Improved sensitization results with either method. Again, recourse to either a solvent or aqueous system will be dictated in large part by the requirements of the processor.

Application of the photoconductive coating composition to the support material can be effected according to standardized methods, well known in the prior art. Thus, for example coating methods such as doctor-blade, swirling, dip-coating and the like may be employed. The thickness to which the photoconductive layer composition is deposited may vary over a relatively wide range; in general, however, wet coating thicknesses within the range of from about .001" to about 0.01" are found to be eminently suitable for accomplishing the purposes of the present invention. Particularly beneficial results are found to obtain with the use of wet-coating thicknesses falling within the range from about .002 to about .006". The support material employed may be any of the conventional materials promulgated in the art for the fabrication of electrostatic recording elements, the principal requirement being that such materials exhibit adequate electrical conductivity. Such materials include, for example, paper (at a relative humidity above about 2%), aluminum-paper laminates, metal foils, such as aluminum foil, zinc foil, etc, metal plates, such as aluminum, copper, zinc, brass, and galvanized plates; regenerated cellulose and cellulose derivatives; certain organic polymeric plastic materials, e.g., polyester and especially polyesters provided with a thin electroconductive layer, such as cuprous iodide coated thereon. Suitable supporting materials include in addition the humidity-independent conducting layers of semiconductors dispersed in polymeric binders.

Other ingredients which may be incorporated into the coating composition for purposes of expediting the coating operation as well as to render the ultimate coating more suitable for use in the image recording process include, for example, plasticizers; e.g., polymeric hydrocarbons having a fair degree of aromaticity and low iodine value; drying agents, e.g., cobalt naphthanate, manganese naphthanate and the like.

The zinc oxide photoconductor materials contemplated for use herein are available commercially. Desirably, the zinc oxide should be provided in the form of relatively small particles having a mean diameter of less than about 0.5 micron. Particularly preferred for use herein is the zinc oxide product produced according to the French Process such as French Process, Florence Green Seal, pigment grade zinc oxide commercially available from the New Nersey Zinc Sales Company Inc. of New York. Other zinc oxide materials preferred for use herein include, for example, the product commercially known as St. Joe PC321 zinc oxide. Optimum realization of the advantages provided by the present invention is obtained by employing the insulating the binder in amounts sufficient to insulate each of the zinc oxide particles from the remaining ingredients of the coating composition. Such proportions can be readily determined by rather routine laboratory investigation.

The recording elements described herein can be advantageously employed with any of the well-known electrophotographic processes based upon the use of photoconductive layers, e.g., the xerographic process the latter being carried out by initially subjecting the electrophotographic element to a blanket electrostatic charge, e.g., by the use of a corona discharge. In view of the insulating character of the photoconductive layer, attributable in the presence of the insulating resin binder material, the uniform charge extant over the surface of the photoconductive layer is retained, such layer also having the property of negligible conductivity in the dark or, as more commonly stated, high dark resistivity. Exposure of the photoconductive layer to light serves to effect an imagewise dissipation of the electrostatic charge from the surface of a layer thus leading to the formation of a latent electrostatic charge pattern. The exposure may be effected through a negative of conventional exposure methods as for example, by contact printing techniques or alternatively by lens projection of an image. The extent of pointto-point charge dissipation depends correspondingly upon point-to-point intensity of the exposure illuminant. The residual charge pattern is thereafter rendered visible or otherwise developed by treatment with a suitable colorant, pigment, etc., comprising electrostatic particles having a charge opposite to that of the residual charge constituting the electrostatic latent image pattern, said developing agent being capable of ready visual comprehension. The developer agent may comprise, for example, a liquid developer in which the developing particles are suspended in an electrically insulating liquid carrier. Developing methods of this type are of course well known being described, for example, in US. Pat. 2,296,691 and in Australian Pat. 212,315. Other developing methods depending upon, for example, heat fusion of resin particles, image transfer are likewise well known in the art and may be utilized to advantage in the practice of the present invention.

The present invention will now be illustrated by reference to the following specific examples. It is to be understood, however, that such examples are presented for purposes of illustration only and the present invention is in no way to be deemed as limited thereto.

EXAMPLE I Preparation of 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimi dinylidene)--[2-( 3 carboxyethyl 2 benzothiazolylidene ethylidene] -3-ethyl-4- oxathiazoline To 3 mmoles of 2-ethyl-4-anilinovinylrhodanine and 3 mmoles of 2-methyl-3-carboxyethylbenzothiazolium iodide in 200 ml. of methanol were added 0.5 ml. of acetic anhydride and 4 ml. triethylamine. The mixture was heated to reflux for 15 to 30 minutes. The reaction mixture was cooled overnight, and filtered. The crystalline precipitate was washed several times with isopropylalcohol and then dried to give the desired merocyanine in yields ranging from 60 to 80%.

Two moles of the above merocyanine were heated with 1.6 grams of methyl-p-toluenesulfonate over a refluxing chlorobenzene bath (B.P. 131 C.) for minutes. After allowing the mixture to cool, 2 moles of thiobarbituric acid, 1020 ml. of methanol and 5 drops of triethylamine were added. The resulting mixture was stirred at room temperature for 30 minutes. The precipitated trinuclear dye was then filtered, washed first with methanol, then with acetone, and finally air dried to yield 0.53 gram of the above identical pure dye exhibiting an absorption maximum in methanol at 585 me. The dye corresponded to the formula:

12 EXAMPLE II An electrophotoconductive coating composition is prepared in the following manner:

To a solution consisting of:

Toluene250 ml.

Cobalt naphthenate0.21 grn. Manganese naphthenate0.21 gm. Xylene5 ml.

is added, with stirring, 151 gms. of Epitex 1311 (a polyester obtained by reacting epichlorohydrin with bisphenol A to form a polyether and thereafter reacting the latter with a mixture of dimerized fatty acid and soya fatty acid as described in US. Pat. 2,970,983). Upon completion 7 of the Epitex addition, 454 gm. of zinc oxide photoconductor (St. Joe PC321) is added to the solution while stirring. With stirring being continued, a resin solution containing 35 gm. of Piccolastic A- (polystyrene resin having a molecular weight of approximately 400 and a melting point of approximately 75 C.) is added. The medium is thereafter stirred and milled until smooth. At this point there is added 20 mg. of the sensitizing dyestuff prepared in Example I, dissolved in 20 ml. of methanol. The medium is thereafter stored for a period of approximately 30 minutes and then coated on Riegel 45 lb. conductive paper to a dried coating thickness of 20 lb. per 3000 sq. ft.

A coated paper is thereafter evaluated electrophotographically by exposure in a Bruning Copytron 2000 the latter comprising commercially available electrophotographic copying apparatus based upon dry toner development. The prints obtained are characterized by excellent density, contrast, etc., and are totally devoid of spurious coloration thus providing high contrast copy. Moreover, such prints exhibited no tendency to discolor upon standing for extended periods of time under varying conditions of heat, humidity, etc.

EXAMPLE III Example II is repeated except that the sensitizing dyestuff is replaced by a substantially equivalent amount of the following materials:

(A) 2- (4,6 dione-2-thione-5-[1H,3H]-pyrimidinylidene)- 5 2- 3 -ethyI-2-benzothiazolylidene) ethylidene] -3 ethyl-4-oxothiazoline (B) 2- (4,6-dione-2-thione-5-[ 1H, 3H] -pyrimidinylidene 5- [2- 3-ethyl-2-benzothiazolylidene ethylidene] -3-,B- carboxy ethyl-4-oxathiazoline (C) 2- (4,6-dione-l-phenyl-Z-thione-S- [3 H] -pyrimidiny1- idene -5- [2- 3 -ethyl-2-benzothiazolylidene) ethylidene -3 -ethyl-4-oxathiazoline (D) 2-(1,3-diethyl-4,6-dione-2-thione-5-pyrimidiny1idene -5- 2- 3-ethyl-2-benzothiazolylidene ethylidene] -3-ethyl-4-oxathiazoline (E) 2- (4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene) 5 [2- 3-ethyl-2-benzoxazolylidene ethylidene] -3-flcarboxyethyl-4-oxathiazoline (F) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene)- 5 2- 3 -ethyl-5 ,6 -dimethyl-2-benzoxazolylidene) ethylidene] -3-ethyl-4-oxathiazoline (G) 2- (4,6-di0ne-2-thione-5-[ 1H,3H] -pyrimidinylidenc 5- 2- 3 -B-carb0xyethyl-6-methoxy-2-benzothiazolylidene ethylidene-3-ethyl-4-oxathiazoline (H) 2- (4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene) 5 2- 3-ethyl-2thiazolidinylidene ethylidene] -3- ethyl-4-oxathiazoline (I) 2- (4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene 5- [2- 3-ethyl-2-thiazolidinylidene) ethylidene] -3- carboxymethyl-4-oxathiazoline (J 2- (4,6-dione-2-thione-5- 1H,3H] -pyrimidinylidene 5 [2- 3-ethyl-5 ,6- dimethyl-2-oxazolylidene -ethylidene] -3-ethyl-4-oxathiazoline (K) 2- 4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene 5- [2- 3 -ethyl-4-pyridylidene ethylidene] -3-ethyl- 4-oxathiazoline (L) 2- (4,6-dione-2-thione-5- 1H,3H] -pyrimidinylidene 2-( 3-ethyl-2-benzimidizolylidene ethylidene] -3-B- carboxyethyl-4-oxathiazoline M) 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene)- 5- 2- 3-,8-carboxyethyl-Z-naphthothiazolylidene) ethylidene] -3 -ethyl-4-oxothiazoline (N) 2- 1 ,3-diphenyl-4,6-dione-2-thione-5-pyrimidinylidene -5- [2- 3-ethyl-2-quinolidene ethylidene] 3- ethyl-4-oxathiazoline (O) 2- 4,6-dione-2-thione-5- 1H,3H] -pyrimidinylidene 5- [2- 3-hydroxyethyl-2-benzothiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline (P) 2- (4,6-dione-2-thione-5-[ 1H,3H] -pyrimidinylidene 5- [2-( 3 ethyl-2-benzothiazolylidene) ethylidene] -3- ,B-ethoxyethyl-4-oxathiazoline (Q) 2-(1,3-diethyl-4,6-dione-Z-thione-S-pyrimidinylidene)-5-(2-(3-ethy1-2-benzothiazolylidene)ethylidene] -3- ,B-methoxy-ethyl-4-oxathiazoline In all cases, substantially equivalent results are obtained, the prints being characterized by excellent density, contrast, etc. In addition, the coated paper showed excellent stability upon aging for protracted periods of time. Also, the dyestulf sensitizers of the present invention were found to have excellent solubility in the usual solvents employed in the production of photoconductive compositions.

While the present invention has been described primarily with respect to the foregoing specific examples, it is to be understood that the present invention is in no way to be deemed as limited thereto but must be construed as broadly as all or any equivalents thereof.

What is claimed is:

1. A photoconductive composition comprising finely divided photoconductive zinc oxide suspended in an electrically insulating film forming vehicle and having incorporated therein a sensitizing dyestuif of the formula:

wherein R and R are selected from the group consisting of lower alkyl, hydroxy lower alkyl, lower alkoxy lower alkyl and carboxy lower alkyl;

R and R are selected from the group drogen, lower alkyl and aryl.

Z represents the nonmetallic atoms necessary to complete a 5- or 6-membered heterocyclic nucleus; and n is an integer of 1 to 2.

2. The composition of claim 1 wherein said sensitizer dyestuff comprises 2 (4,6 dione-2-thione-5-[1H,3H]- pyrimidinylidene) 5 [2 (3 B carboxyethyl 2 benzothiazolylidene)ethylidene] 3 ethyl 4 oxathiazoline.

consisting of hy- 3. The composition of claim 1 wherein said sensitizer dyestuff comprises 2-(4,6-dione-2-thione-5-[lH,3H]-pyrimidinylidene)-5-[2-(3-ethyl 2 benzothiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline.

4. The composition of claim 1 wherein said sensitizer dyestuff comprises 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene)-5-[2-(3-ethyl 2 benzothiazolylidene) ethylidene] -3fi-carboxyethyl-4-oxathiazoline.

5. The composition of claim 1 wherein said sensitizer dyestuff comprises 2- (4,6-dionel-phenyl-Z-thione-S- 3H] pyrimidinylidene)-5-[2-(3-ethyl 2 benzothiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline.

6. The composition of claim 1 wherein said sensitizer dyestuif comprises 2- 1,3-diethyl-4,6-dione-2-thione-5-pyrimidinylidene)-5-[2-(3-ethyl 2 benzothiazolylidene) ethylidene] -3-ethyl-4-oxathiazoline.

7. The composition of claim 1 wherein said sensitizer dyestuif comprises 2-(4,6-dione-2-thione-5-[lH,3H]-pyrimidinylidene)-5-[2-(3-ethyl 2 benzoxazolylidene) ethylidene] -3 8-carboxyethyl-4-oxathiazoline.

8. The composition of claim 1 wherein said sensitizer dyestuif comprises 2-(4,6-dione-2-thione-5-[ lH,3H]-pyrimidinylidene)-5-[2-(3-ethyl 5,6 dimethyl-Z-benzoxazolylidene) -ethylidene] -3-ethyl-4-oxathiazoline.

9. The composition of claim 1 wherein said sensitizer dyestutf comprises 2-(4,6-dione-2-thione-5-[lI-I,3H]-pyrimidinylidene-S-[2-(3-ethyl 5,6 dimethyl-Z-benzoxazolylidene -ethylidene] -3 -ethyl-4-oxathiazoline.

10. The composition of claim 1 wherein said sensitizer dyestufr comprises 2-(4,6-dione-2-thione-5-[1H,3H]-pyrimidinylidene-S-[2-(3-ethyl 2 thiazolidinylidene)ethylidene] -3-ethyl-4-oxathiazoline.

11. The composition of claim 1 wherein said sensitizer dyestuff comprises 2-(4,6-dione-2-thione-5-[lH,3H]pyrimidinylidene)-5-[2-(3-ethy1 2 thiazolidinylidene) ethylidene] -3-carboxymethyl-4-oxathiazoline.

12. The composition of claim 1 wherein said sensitizer dyestuff comprises 2-(4,6-dione-2-thione-5-[ 1H,.3H] -pyrimidinylidene-5-[4-(l-phenyl 3 methyl-5-pyrozolan-4- ylididene methyl] -3 -ethyl-4-hydroxy-2-thiazolylidene.

References Cited US. Cl. X.R. 260240.4 

